Immediately, some theories developed: Too much offseason work. Not enough. New labor-contract rules limiting padded practices to one per day, while generally seen as helpful, are hardly a cure-all.

Washington Redskins linebacker London Fletcher thinks some guys get hurt in camp because players are trying so hard to impress coaches and earn a roster spot or a starting job.

"You know now coaches are really evaluating you," said Fletcher, whose teammate, second-year linebacker KeenanRobinson, tore his left pectoral muscle on Day 1 of training camp. "You've got guys with a competitive spirit and they're looking at it, like, 'My job's on the line. I need to make a play' and not realizing there's going to be times to show that coaches that you can hit, you can make plays in preseason games, but you don't want to have a guy go down because of something that happened in practice."

Whatever the cause, severe injuries are increasing in the NFL. The number of injuries that forced a player to miss at least eight days jumped every year from 2009 to 2012, according to an analysis of NFL injury data released Wednesday.

The study by Edgeworth Economics, based on information collected by the league, also shows that players with concussions missed an average of 16 days last season, up from only four days in 2005, while the length of time out for other types of injuries has been steadier.

The study says there were 1,095 instances of injuries sidelining a player for eight or more days in 2009 -- including practices and games in the preseason, regular season and postseason -- and that climbed to 1,272 in 2010, 1,380 in 2011, and 1,496 in 2012. That's an increase of 37 percent.

"The way I look at it, really, is that injuries are part of the game," said cornerback Kyle Wilson of the New York Jets, who lost another cornerback, Aaron Berry, for the season when he tore a knee ligament on the first day of practice last week. "Injuries happen sometimes. They're unfortunate, but it really is just part of the game."

Concussions have become a far-more-noticed part of football in recent years, with more discussion of the links between head injuries and brain disease, hundreds of lawsuits brought by thousands of former players, and rules changes made by the NFL to try to better protect players.

During the nine years examined in David's study, the average number of days missed because of head injuries by players in the league went from 4.8 in 2004, four in 2005, and 4.1 in 2006, to 10.9 in 2010, 12 in 2011, and 16 last season.

"We have experts at practice every day to let you know, as a coach, if someone does have a concussion, so that makes it pretty easy. They leave it out of our hands; they put in the experts' hands," Redskins coach Mike Shanahan said. "But, yeah, I think there's more awareness in a lot of different areas when it comes to injuries over the last few years, and rightfully so."

David said "you now have more severe injuries overall" because of the hike in lengthy absences for reported concussions.

"Are the brain injuries actually more severe now than they were five years ago? Or is that players simply being held out longer for the same injury? That we can't tell from the data," David said. "My guess is it's both, but how much of each factor, I don't know."

NFL spokesman Brian McCarthy, who said the league will look at the study's findings, attributed the longer absences for players with concussions to more caution in the treatment of those types of injuries.

"We do know that the game is safer now, but we still have work to do. We continue to work hard on many fronts to make the game better and safer for our sport at all levels," McCarthy wrote in an email. "Our ongoing efforts include making rule changes designed to take dangerous techniques out of the game and also improving medical care to properly manage and treat concussions and raise awareness of their seriousness."